It wouldn't have to be complex, either, and it certainly isn't useless (says me and a few other people already!).

Quick example: I recently typed out about 10 long lines of business development chatter in Slack, the first of which contained a list of prospects. I realized I left a few out in my list, and edited the message to add them in. Without editing, I'd have to have either posted the complete list again or added them in as a second comment. Editing the original message is simply a lot less confusing.

With editing if people read the first version of your opening message, you don't know if they saw the edits or not.

I find edits useful for typos/formatting/cosmetics, but I don't ever edit significant information into a previous post for that reason.

I'm not sure why your comment was down voted, but as both an IRC and Slack user, I have to say I love Slack's built in pastebin.

I do wonder though, should IRC evolve to be like Slack or should it evolve idependently? I feel that the new features should be more vital, rather than "neat"... like maybe more focus on privacy and/or anonymity. Undoubtedly, I am just resisting change...

I see chat like in-person communication where editing something already said is impossible.

Once you try it, you never want to go back.